As is known, magneto-inductive flow sensors enable measurement of the volume flow, e.g. volume flow rate, of an electrically conductive fluid flowing in a stream direction through a flow sensor measuring tube. For this, a magnetic field of highest possible density is produced at the flow sensor by means of a magnetic circuit arrangement coupled to an exciter electronics. The magnetic field passes through the fluid within a measuring volume at least sectionally perpendicularly to the flow direction. The magnetic field closes on itself essentially outside of the fluid. The measuring tube therefore is usually made of a non-ferromagnetic material, in order that the magnetic field is not unfavorably influenced during measuring.
As a result of the movement of free charge carriers of the fluid in the magnetic field according to the magneto-hydrodynamic principle, there is produced in the measuring volume an electric field, which is directed perpendicular to the magnetic field and perpendicular to the flow direction of the fluid. By means of at least two measuring electrodes arranged spaced from one another in the direction of the electric field, and by means of an evaluation electronics connected to these electrodes, an electric voltage induced in the fluid is measurable. This voltage is a measure for the volume flow rate. The flow sensor is so constructed, that the induced electric field closes outside of the fluid essentially exclusively by way of the evaluation electronics connected to the measuring electrodes.
Measuring electrodes, for example, galvanic, fluid-contacting, or capacitive, fluid-non-contacting, measuring electrodes, can serve for tapping the induced voltage.
For guiding the magnetic field and for in-coupling of the magnetic field into the measuring volume, the magnetic circuit arrangement usually includes two coil cores, which are arranged spaced from one another, especially diametrally spaced, on the periphery of the measuring tube. Each core includes a free end face and these faces are located especially as mirror images of one another. The magnetic field is so coupled into the coil cores by means of a coil arrangement connected to the exciter electronics, that it passes through the fluid in the measuring tube at least sectionally perpendicularly to the flow direction of the fluid.
Due to the required, high mechanical stability of such measuring tubes, they are made preferably of an outer, especially metal, carrier tube of predeterminable strength and diameter, and the carrier tube is coated internally with an electrically non-conductive, insulating material of predeterminable thickness, the so-called liner. Thus, in U.S. Pat. No. 6,595,069, U.S. Pat. No. 5,280,727, U.S. Pat. No. 4,253,340, U.S. Pat. No. 3,213,685 or JP-Y53-51 181, a magneto-inductive flow sensor is described, which includes:                a measuring tube, which is insertable pressure-tightly into a pipeline and has a first, inlet end and a second, outlet end,                    having a non-ferromagnetic carrier tube as an outer encasement of the measuring tube, and            a tubular liner accommodated in a lumen of the carrier tube, the liner being of an insulating material and serving to convey a flowing fluid insulated from the carrier tube,                        a magnetic circuit arrangement arranged on the measuring tube for producing and guiding a magnetic field, which induces an electric field in the flowing fluid, as well as        a first measuring electrode and a second measuring electrode for the tapping of a voltage of the electric field.        
The liner serves for the chemical isolation of the carrier tube from the fluid. In the case of carrier tubes of high electric conductivity, especially in the case of metal carrier tubes, the liner serves, moreover, as an electrical insulation between the carrier tube and the fluid, which prevents a short circuiting of the electric field through the carrier tube. By a corresponding design of the carrier tube, therefore, a matching of the strength of the measuring tube to the mechanical loads present in the particular application is implementable, while, by means of the liner, a suiting of the measuring tube to the chemical, especially hygienic, requirements present for the particular application can be realized. For manufacturing the liner, often injection molding or transfer molding methods are used. It is, however, also usual to insert into the carrier tube a completely prefabricated liner. Thus, in JP-A 59-137 822, a method is disclosed, in which the liner is formed from softened plastic foil.
In the liner, made most often of a thermoplastic, or thermosetting, plastic, usually open-pored support skeletons are embedded to give it stability; compare, for example, also EP-A 36 513, EP-A 581 017, JP-Y 53-51 181, JP-A 59-137 822, U.S. Pat. No. 6,595,069, U.S. Pat. No. 5,664,315, U.S. Pat. No. 5,280,727 or U.S. Pat. No. 4,329,879. These serve to stabilize the liner mechanically, especially relative to pressure changes and thermally related, volume changes. For example, in U.S. Pat. No. 5,664,315, a method is described for manufacturing a measuring tube of a magneto-inductive flow sensor, which is provided internally with a liner, wherein, before the installing of the liner into the carrier tube, an expanded metal lattice in the form of a prefabricated support skeleton is put in place for mechanically stabilizing the liner. Additionally, in JP-Y 53-51 181, a tubular support skeleton is disclosed, in whose lateral surfaces bores are formed, while in EP-A 581 017 or U.S. Pat. No. 6,595,069, sintered support skeletons are shown. The support skeletons are placed in the measuring tube in alignment therewith and are completely encased by the insulating material, at least on the inner side contacting the fluid
Further, in U.S. Pat. No. 6,595,069, a method for manufacturing a liner with embedded support skeleton is disclosed, wherein support skeleton and liner are manufactured directly in the lumen of the carrier tube, with the support skeleton being first formed by sintering within the carrier tube and the liner being subsequently formed by solidification of liquid insulating material charged into the carrier tube.
By the sintering of the support skeleton directly in the carrier tube, such can be matched in an almost completely flexible manner in its form and size to the requirements set by the application or also by the manufacturing process. For example, it has also been disclosed in U.S. Pat. No. 6,595,069, that the support skeleton is formed in each case with the ends broadening such that it fits in correspondingly end-located, funnel-shaped widenings in the carrier tube, and is, in this way, axially fixed in place. Beyond this, a support skeleton is shown in U.S. Pat. No. 6,595,069, which completely or partially fills lateral openings provided centrally in the carrier tube, so that an additional locking of the support skeleton in the carrier tube is achieved.
It has been shown, it is true, on the one hand, that liners of the described kind exhibit a very high mechanical durability, even in temperature ranges of −40° C. up to 200° C. On the other hand, however, this high durability is associated very closely with an extremely high quality, especially also of the support skeleton.
Investigations have, however, shown, that, as a result of the, at times arising, high temperatures of up to 1000 K (Kelvin), especially during the sintering of the support skeleton and during casting of the liner, and, because of the usually mutually differing cooling behavior of carrier tube and support skeleton, resulting from their mutually differing thermal, material properties and forms, high mechanical stresses can arise in the support skeleton, stresses which, under the right circumstances, can lead to crack formation and thus to a reduction of the quality of the support skeleton, or, correspondingly, even to the destruction of the liner. Additionally, it has been determined, that the almost unavoidable shrinkage of the sinter material following the sinter process can lead to a considerable play between support skeleton and carrier tube. Additionally, it was determined that such a play, for example in the case of the support skeleton disclosed in U.S. Pat. No. 6,595,069, can lead to a no longer negligible or tolerable shifting of the same in the carrier tube, so that additional measures must be used for centering and locking the support skeleton in the carrier tube.